Study of Carrier Generation in Phthalocyanines by Time-Resolved Fluorescence

We have studied the influence of electric field on fluorescence in particles of photoconductive TiOPc (I), TiOPc (IV), HOGaPc, and x-H2Pc dispersed in a polymer matrix. Electric field induced quenching of both the integrated and time-resolved fluorescence were measured. Time-resolved fluorescence decays were analyzed by fitting the data to a sum of two exponentials, representing the fast and slow fluorescence components. For HOGaPc, TiOPc(I), and TiOPc (IV), the fast fluorescence component exhibits both amplitude and lifetime quenching. These results indicate that carrier generation in HOGaPc, TiOPc(I), and TiOPc (IV) originates from both relaxed and nonrelaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. In contrast, for x-H2Pc, significant amplitude quenching of the fast component is observed only at high field, and the trapped excitons are an important source of photogenerated carriers. This indicates that x-H2Pc possesses at least some bulk-sensitized photocarrier generation. All of the phthalocyanines studied exhibited a quadratic dependence of integrated fluorescence quenching on electric field, indicating the existence of a neutral carrier precursor state.